Hypromellose acetate succinate is a widely known enteric polymer. It is a polymer obtained by introducing four kinds, in total, of substituents to the cellulose backbone. More specifically, the hypromellose acetate succinate has ether structures formed by introduction of two substituents, a methyl group (—CH3) and a hydroxypropyl group (—C3H6OH), and ester structures formed by introduction of two substituents, an acetyl group (—COCH3) and a succinyl group (—COC2H4COOH).
The hypromellose acetate succinate (hereinafter also referred to as “HPMCAS”), which is an enteric polymer, has been widely used for a solid dispersion for improving the dissolution properties of a poorly water-soluble drug, and for enteric coating.
The solid dispersion is obtained, for example, by solidification through hot melt extrusion of a poorly water-soluble drug and the polymer. Alternatively, attention is now paid to a method such as spray-drying. The spray-drying for producing a solid dispersion comprises the steps of: dissolving a mixture of a drug and the polymer in a solvent and then removing the solvent for deposition. For example, a solid dispersion obtained by spray-drying a solution of a poorly water-soluble drug and the polymer has improved bioavailability because the drug is molecularly dispersed in amorphous form in a polymer carrier to markedly and seemingly increase a solubility.
An enteric coating preparation is one of important preparations widely used for administration of an acid-labile drug or for protection of the gastric mucosa. In conventional production of an enteric coating preparation, it is the common practice to use a method comprising the steps of: dissolving an enteric polymer in an organic solvent, and spraying the resulting solution to form an enteric film on the surface of a drug. A so-called aqueous enteric coating method using an aqueous dispersion of a finely pulverized enteric polymer has been developed in consideration of environmental conservation or safety against use of an organic solvent (JP 07-109219A). For example, there is reported an ammonia-neutralized coating method using an aqueous enteric coating solution obtained by dissolving HPMCAS, which is an enteric polymer, in such an amount of ammonia as to be required for neutralizing about 80 mol % or more of the carboxyl group in the molecule of the HPMCAS (JP 08-245423A).